In a telecommunications system, signalling equipment and signalling channels are required for the exchange of information between system nodes. In particular, this internode signalling informs traffic channel switches of what is to be performed when a telephone or data call is to be set up or released in so-called “circuit-switched” connections. Signalling is also used to query centrally located databases, e.g. to obtain routing information for 800 numbers and to determine the location of a subscriber in a Public Land Mobile Network (PLMN).
Modern telecommunications systems now largely make use of Common Channel Signalling (CCS) whereby signalling information is transmitted on one or more dedicated signalling channels, distinct from the channels used to carry actual user information (e.g. voice or data). An important feature of CCS is that the same signalling system may support services in a variety of existing telecommunications protocols, e.g. Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), and Public Land Mobile Networks (PLMN), as well as proposed future protocols such as B-ISDN, enhancing greatly the interoperability of networks supporting different protocols.
Currently, the predominant CCS is known as Signalling System Number 7 (SS7), defined in the ITU-T (International Telecommunications Union—Telecommunications section) recommendations starting with Q.700, and by ANSI (American National Standards Institute) in recommendations T1.111. SS7 is a packet switched system having multiple signalling links of one time slot in a Time Division Multiple Access (TDMA) E.1 or T.1 transmission format (the other time slots being available for user data). Individual signalling message packets (datagrams) are associated with respective individual telephone calls. As only a relatively small amount of signalling information is associated with a single telephone call, a single SS7 channel is able to handle all signalling between two network nodes (termed “signalling points”) for several thousands of calls.
As already alluded to above, SS7 is able to provide a signalling message transport mechanism for a number of different applications. FIG. 1 illustrates an SS7 protocol stack of which the lowest level, Message Transfer Part (MTP) level 1, defines the physical, electrical, and functional characteristics of a digital signalling link. MTP level 1 has a number of different possible forms including the European standard E.1 (2048 kb/s and 32 64 kb/s channels). MTP level 2 takes care of the accurate end-to-end transmission of messages across a chosen signalling link whilst MTP level 3 handles the routing of signalling messages between neighbouring signalling links based upon information received from higher SS7 levels concerning the final destination of a signalling message. MTP level 3 handles inter alia re-routing of messages away from failed or congested signalling links.
Above the MTP levels, SS7 comprises an ISDN User Part (ISUP) which defines the protocol and procedures involved in setting-up, controlling, and tearing-down circuit switched connections which carry voice and data over the Public Switched Telephone Network (PSTN). ISUP is not only used in ISDN networks, but is also employed in non-ISDN networks. A Telephone User Part (TUP) supports basic call processing for analogue calls and is used for example in China.
With the development of advanced network functions such as freephone (800 numbers), call forwarding, mobile roaming, etc, it has been necessary to add to SS7 a number of additional levels to support these functions. In particular SS7 is now provided with a Signalling Connection Control Part (SCCP) which handles the routing of signalling messages to and from the numerous applications which use the SS7 transport mechanism. SCCP also provides for the transformation of a Global Title (e.g. a dialled 800 number) into a destination signalling point (in Europe a signalling point is defined by a combination of a Network Indicator and a Signalling Point Code).
Located above the SCCP is a Transaction Capabilities Application Part (TCAP) which is a protocol for dealing with the exchange of information between signalling points related to database queries. For example, a switch within a network may use TCAP to determine the B-number associated with a dialled 800 number. This may require the sending of a suitable TCAP message to a database associated with the 800 number, with the SCCP performing the identification of signalling point at which the database is located.
The applications which make use of TCAP, and by extension SCCP, are many. For example the Mobile Application Part (MAP) is used in PLMNs to transfer information between Mobile Switching Centres (MSCs), Home Location Registers (HLRs), and Visitor Location Registers (VLRs), whilst the Intelligent Network Application Part (INAP) is used to transfer information between intelligent network nodes and telephone switches.
It will be appreciated that SS7 has evolved into a complex and hence computationally intensive set of protocols. There therefore exists, a desire to both reduce this complexity and also to reduce the high costs of signalling networks (both in terms of infrastructure and of maintenance). Telecommunications network designers have turned for an answer to the field of data networks and in particular to Internet Protocol (IP) networks where high demand has lead to relatively low cost and technically advanced solutions.
Several proposals have been put forward to employ IP networks in telecommunications signalling. For example, it has been proposed to encapsulate TCAP messages into IP datagrams for transmission over an IP network, with a mapping being made between Global Titles (and/or destination signalling points) and IP addresses for the purpose of routing the datagrams.